I don't think Carl got fat and happy. He was trying to win a Super Bowl with Vermeil as fast as possible before Lamar died.
After 2005 everything went south. Was he fat and happy? No, he just made two horrible choices in Herm and Croyle.
Alex Smith's current 9.8 YPC is one of the lowest marks in the NFL over the last 10 seasons. Only a few QBs have been worse:
Ryan Mallett (9.8), Derek Carr (9.4), Christian Ponder (9.8), Brady Quinn (9.8), Ryan Fitzpatrick (8.6), Brodie Croyle (9.7), Charlie Frye (9.7), Bruce Gradkowski (9.4), David Carr (9.2)